This invention relates to systems or methods for positioning one or multiple xe2x80x9ctools,xe2x80x9d such as laser beams or other radiation beams, relative to target locations on one or multiple workpieces and, in particular, to a system that accurately compensates for Abbe errors associated with the movement of one or more stages of such a beam positioning system.
A variety of technologies employ tools to micro-machine, or deposit patterns or materials on target locations on a workpiece. For example, a micro-dimensioned punch may be used to punch holes in a thin metal plate; a laser may be used to precisely machine or selectively erode metallic, crystalline, or amorphous specimens; and ion beams may be used to selectively implant charged particles into an integrated circuit. All of the above-mentioned processes share a common requirement for accurately and rapidly positioning a pertinent tool to target locations on the workpiece.
The following background is presented herein only by way of example to laser beam positioning systems, but skilled persons will appreciate that the description is applicable to tool positioning systems in general. Conventional tool positioning systems, and particularly beam-positioning systems, typically provide movement within a three-dimensional coordinate system and can be characterized in several ways.
Traditional positioning systems are characterized by X-Y translation tables in which the workpiece is secured to an upper stage that is supported by a lower stage. Such systems typically move the workpiece relative to a fixed beam position and are commonly referred to as stacked stage positioning systems because the lower stage supports the inertial mass of the upper stage and the workpiece. These positioning systems have relatively good positioning accuracy because interferometers are typically used along each axis to determine the absolute position of each stage.
In U.S. Pat. No. 4,532,402 of Overbeck, a high-speed short-movement positioner (xe2x80x9cfast positionerxe2x80x9d), such as a galvanometer, is supported by the upper stage of an X-Y translation table (xe2x80x9cslow positionerxe2x80x9d) and the upper stage and the workpiece are supported by the lower stage. The combined movement of the two positioners entails first moving the slow positioner to a known location near a target location on the workpiece, stopping the slow positioner, moving the fast positioner to the exact target location, stopping the fast positioner, causing the tool to operate on the target location, and then repeating the process for the next target location.
However, the combined system of Overbeck is also a stacked stage positioning system and suffers from many of the same serious drawbacks as the aforementioned fixed beam system. The starting, stopping, and change of direction delays associated with the inertial mass of the stages and fast positioner unduly increase the time required for the tool to process the workpiece. Overbeck""s system also imposes a serious drawback upon a computer-based machine tool control file or xe2x80x9cdatabasexe2x80x9d that typically commands the tool to move to a series of predetermined target locations across the workpiece. The database positioning the tool across the workpiece must be xe2x80x9cpanelizedxe2x80x9d into abutting segments that each fit within the limited movement range of the fast positioner when the size of large circuit patterns exceeds this movement range.
U.S. Pat. Nos. 5,751,585 and 5,847,960 of Cutler et al. describe split-axis positioning systems, in which the upper stage is not supported by, and moves independently from, the lower stage and in which the workpiece is carried on one axis or stage while the tool is carried on the other axis or stage. These positioning systems have one or more upper stages, which each support a fast positioner, and can process one or multiple workpieces simultaneously at high throughput rates because the independently supported stages each carry less inertial mass and can accelerate, decelerate, or change direction more quickly than can those of a stacked stage system. Thus, because the mass of one stage is not carried on the other stage, the resonance frequencies for a given load are increased. Furthermore, the slow and fast positioners are adapted to move, without necessarily stopping, in response to a stream of positioning command data while coordinating their individually moving positions to produce temporarily stationary tool positions over target locations defined by the database. These split-axis, multirate positioning systems reduce the fast positioner movement range limitations of prior systems while providing significantly increased tool processing throughput and can work from panelized or unpanelized databases.
Such split-axis positioning systems are becoming even more advantageous as the overall size and weight of the workpieces increase, utilizing longer and hence more massive stages. At the same time, feature sizes are continuing to decrease, causing the need for dimensional precision to increase, and split-axis systems are more likely to exhibit rotational errors that introduce Abbe errors, which are errors indicative of the physical separation between the effective position of a stage and the indicated position of the stage. Abbe errors are typically caused by imperfections or thermal variations in the bearings upon which the stages slide and/or alignment or acceleration imperfections of the drive mechanisms that provide movement to the stages.
FIG. 1 shows three mutually perpendicular translational motion axes, such as X axis 10, Y axis 12, and Z axis 14 that define a three-dimensional coordinate system 16, and three mutually perpendicular rotational motion axes (hereafter referred to as a roll axis 18, a pitch axis 20, and a yaw axis 22). Skilled workers typically refer to roll as an angular rotation about X-axis 10, pitch as an angular rotation about Y-axis 12, and yaw as an angular rotation about Z-axis 14.
Although laser interferometer systems can be used to indicate and compensate for certain Abbe errors, such systems are costly and heavy because they typically require reference mirrors that are nearly as long as the combined stage length plus the length of travel, e.g. as much as two times the travel distance. Such mirrors are difficult, if not impossible, to procure for the long travel dimensions of large stages, such as with a lengthwise dimension of 76 to 92 cm (30-36 inches), needed to accommodate larger workpieces. Furthermore, split-axis systems would require at least two interferometers for each stage and/or a very complex system of optics to indicate angle and position, and the additive weight of the interferometers would increase the inertial load on the stages at the expense of frequency response time to changes in momentum.
U.S. Pat. No. 5,699,621 of Trumper et al. discloses the use of small range displacement transducers to indicate pitch, yaw, and roll angle errors. Trumper et al. correct angular errors by controlling the bearing gap with electromagnets that require the use of a highly compliant magnetic or air bearing system. The correction speed of the Trumper et al. system is limited to the bandwidth of the linear stage system and therefore has similar mass versus bandwidth limitations as stacked stage positioning systems.
A less expensive and/or less massive and very accurate Abbe error correction system or method is therefore desirable.
An object of the present invention is to provide a method or apparatus that employs non-contact small displacement sensors, such as capacitive sensors, to determine Abbe errors due to mechanical stage pitch, yaw and roll that are not indicated by an on-axis position indicator, such as a linear scale encoder or laser interferometer, and a means to compensate for such Abbe errors.
Another object of the invention is to employ such sensors to determine and correct Abbe errors due to linear bearing variability or distortions associated with acceleration or temperature gradients.
The present invention provides a cost effective means to determine and compensate for linear stage positioning system Abbe errors that are errors at the effective position of the system that are not indicated by a position indicator such as a metal or glass scale encoder or laser interferometer due to pitch, yaw, or roll of the linear stage and the resulting physical distance between the effective position and the indicated position of a stage. To minimize cost, the system is calibrated against precision X and Y position reference standards so the corrections depend only on sensing small changes in the sensor readings and not on absolute accuracy of the sensor readings. Although the present invention is preferred for use in split-axis positioning systems, it can be employed in stacked stage systems to reduce their manufacturing costs. Although a linear scale encoder can be employed to indicate the nominal on-axis stage position to reduce costs further, a laser interferometer can be used when a greater level of accuracy and/or resolution is desired.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.